1. Field of the Invention
The inventions disclosed and taught herein relate generally to compositions and methods for controlling the gelation rate in aqueous-based fluids useful in treating subterranean formations. More specifically, the present disclosure is related to improved compositions for use in the controlled gelation, or crosslinking, of polysaccharides in aqueous solutions with sparingly-soluble borates, as well as methods for their use in subterranean, hydrocarbon-recovery operations.
2. Description of the Related Art
Many subterranean, hydrocarbon-containing and/or producing reservoirs require one or more stimulation operations, such as hydraulic fracturing, in order to be effectively produced. Borates were some of the earliest crosslinking agents used to increase the viscosity and proppant-transport capabilities of aqueous, guar-based stimulation fluids, and have been used successfully in numerous low- to moderate-temperature (<200° F.) reservoirs. However, as hydrocarbon exploration capabilities expanded, the number of subterranean reservoirs being developed with temperatures greater than 200° F. increased, the conventional borate-salts used, and the resulting crosslinked fluids, were found to provide inadequate rheological stability.
Thus, as the development of high-temperature (>200° F.) well stimulation fluids were developed, an emphasis was placed on the maximization of the thermal stability of the rheological properties of the fluids. In particular, titanium and zirconium crosslinking agents were developed for their ability to provide stable, somewhat controlled, bonding in high-temperature subterranean environments.
Fracturing fluids that are crosslinked with titanate, zirconate, and/or borate ions (using compounds which generate these ions in the fluid), sometimes contain additives that are designed to delay the timing of the crosslinking reactions. Such crosslinking time delay agents permit the fracturing fluid to be pumped down hole to the subterranean formation before the crosslinking reaction begins to occur, thereby permitting more adaptability, versatility or flexibility in the fracturing fluid. Additionally, the use of these gelation control additives can be beneficial from an operational standpoint in completion operations, particularly because their use allows for a decrease in the amount of pressure required for pumping the well treating fluids. This in turn can result in reduced equipment requirements and decreased maintenance costs associated with pumps and pumping equipment. Examples of early crosslinking time delay agents that have been reported and have been incorporated into water-based fracturing fluids include organic polyols, such as sodium gluconate, sodium glucoheptonate, sorbitol, glyoxal, mannitol, phosphonates, and aminocarboxylic acids and their salts (EDTA, DTPA, etc.).
A number of additional classes of previously used delay additives and compounds for use in controlling the delay time and the ultimate viscosity of treating fluids, such as fracturing fluids, have been previously reported. As can be imagined, the gelation control additives and methods vary, depending upon whether the crosslinking agent is a borate-based crosslinker or a transition metal crosslinker (e.g., Zr or Ti). Generally, the agents used to slow the crosslinking of guar and guar-type fluids are polyfunctional organic materials which have chelating capabilities and can form strong bonds with the crosslinking agent itself. Several classes of agents have been described to date, especially for the controlled crosslinking by zirconium and titantium. For example, a hybrid delay agent having the trade name TYZOR® (DuPont) for the delay of viscosity development in fracturing fluids based on guar derivatives crosslinked with a variety of common zirconate and titanate crosslinkers under a wide pH range and under a variety of fluid conditions has been described by Putzig, et al [SPE Paper No. 105066, 2007]. Other delay agents for such organic transition-metal based crosslinkers include hydroxycarboxylic acids, such as those described in U.S. Pat. Nos. 4,797,216 and 4,861,500 to Hodge, selected polyhydroxycarboxylic acid having from 3 to 7 carbon atoms as described by Conway in U.S. Pat. No. 4,470,915, and alkanolamines such as triethanolamine-based delay agents available under the trade name TYZOR® (E.I. du Pont de Nemours and Co., Inc.). However, the use of many of these transition-metal based crosslinkers, and their often-times costly crosslink time delay additives have occasionally been associated with significant damage (often greater than 80%) to the permeability of the proppant pack when used in hydraulic fracturing operations, especially in formations having elevated temperatures [Penny, G. S., SPE 16900 (1987); Investigation of the Effects of Fracturing Fluids Upon the Conductivity of Proppants, Final Report, (1987) STIM-LAB Inc. Proppant Consortium (1988)].
A number of approaches to the control of the crosslinking process in fluids comprising fully-soluble borate crosslinkers have also been described. For example, a number of polyhydroxy compounds such as sugars, reduced sugars, and polyols such as glycerol have been reported to be delay agents for crosslinkers based on boron. Functionalized aldehyde-based and dialdehyde-based delay agents for fully-soluble borates, such as those described in U.S. Pat. Nos. 5,082,579 and 5,160,643 to Dawson, have also been reported. However, numerous of these gelation control agents for use in boron-based crosslinker compositions are highly pH and temperature dependent, and cannot be used reliably in subterranean environments having elevated pHs, e.g., a pH greater than 9 and/or temperatures greater than about 200° F.
The mechanism for delay in crosslinking time of organic polymer in fluids comprising sparingly-soluble borate-based crosslinkers has also been documented to some extent. As was described in U.S. Pat. No. 4,619,776 to Mondshine, the unique solubility characteristics of the alkaline earth metal borates or alkali metal alkaline earth metal borates enables them to be used in the controlled crosslinking of aqueous systems containing guar polymers. The rate of crosslinking could be controlled by suitable adjustment of one or more of the following variables—initial pH of the aqueous system, relative concentrations of one or more of the sparingly-soluble borates, temperature of the aqueous system, and particle size of the borate. However, there are several limitations in the aforementioned art for sparingly soluble borates which are incorporated in water-base crosslinking suspensions for fracturing operations—particle size/concentrations of the borate solids, and the initial pH of the guar solution.
At present, the primary method for varying crosslink times of a treatment fluid utilizing sparingly soluble borate is with modification of the borate particle size alone. Operational requirements for delayed crosslink times as fast as 30-45 seconds have not been accomplished with present technology. Smaller particles may sometimes decrease crosslink times, but even with milling and air classification, the size is often not sufficiently fine or small enough to produce the desired rapid crosslink times. Additionally, limited solubility borate solids exhibit a major change as the pH of the base guar solution is changed. For example, when the alkalinity is incrementally increased from a more acidic pH to a basic pH 10.0, the crosslink time is faster. At pH values greater than about pH 10.0, the crosslink time reverses and becomes slower as the alkalinity is increased. As a result, higher pH values (e.g., about 11.6) which are utilized to provide gel viscosity stability at elevated temperatures exhibit crosslink times greater than 12 minutes even with very fine borate solids. Accelerating crosslink times using finer particles with more surface area, or increased concentrations of sparingly-soluble borate is not feasible due to gelation of the crosslinking concentrate caused by more solids and their subsequent interaction.
In view of the above, the need exists for compositions, systems, and methods for providing more precise control of delays over the crosslinking reaction of borated aqueous subterranean treating fluids, such as fracturing fluids. The inventions disclosed and taught herein are directed to improved compositions and methods for the selective control of the rates of crosslinking reactions within aqueous subterranean treating fluids, especially at varying pH and over a wide range of formation temperatures, including formation temperatures greater than 200° F.